Light Emitting Diodes (LEDs) are extremely durable and efficient sources of light. Recent developments in LED technology have resulted in components that can be combined into arrays having a brightness and light emission pattern comparable with light sources such as halogen bulbs and xenon flash tubes. Among other attractive characteristics such as durability and energy efficiency, LEDs have a very fast turn-on time of approximately 20 μS that is comparable with the turn on time of a xenon flash tube. If properly energized, an LED array can simulate the light-emission pattern of any pre-existing light source, including a xenon flash tube.
LEDs are solid-state components whose light output (luminous flux) increases in proportion to the applied forward current. However, excessive current through an LED generates heat that can damage the device. Each LED develops a forward voltage VF that varies with the color of the LED as well as the ambient temperature. LED manufacturers typically rate each type of LED for continuous operation at an average forward current IAvg. Each type of LED is also rated for momentary operation at a peak pulsed forward current IPeak. Application of IPeak to an LED produces increased brightness when compared to an LED energized by application of IAvg. This peak brightness is particularly desirable in warning lights used in conjunction with emergency vehicles, aircraft, traffic signaling, etc. However, continuous application of IPeak would result in excessive heat and damage or failure of the LED. There is a need in the art for LED driver circuits that permit precise control of the pattern and quantity of current applied to an LED array to produce a light emission pattern of maximum intensity without overheating the LEDs.
It is desirable that each LED driver circuit generate the requisite controlled current over a range of output voltages to accommodate variation not only in the ambient temperature and the type of LED employed but also variation of the number of LEDs being driven. Constant current sources suitable for such an application are known in the art. One approach is to employ a chip called a switching regulator to control the applied current by varying the duty cycle of energy applied to the LEDs. The switching regulator is responsive to a current sensing circuit to control the applied current. In this manner, circuits can be configured to apply controlled currents to a series array of LEDs over a range of output voltages. For example, a switching regulator configured as a buck converter circuit may be used to produce controlled current over a range of voltages less than the available input voltage, while a boost converter may be used to produce controlled current over a range of voltages greater than the available input voltage.
Maximum flexibility in terms of output voltage range allows a single driver circuit (also referred to as a ballast) to be used in conjunction with different numbers of LEDs of different types and over a range of ambient temperatures. Those of skill in the art will recognize that the design and manufacture of a driver circuit for each individual LED in an array or for each array having a particular number of LEDs is not cost-efficient. Therefore, there is a need in the art for multi-purpose LED driver circuits for energizing LED light sources that incorporate different numbers and/or types of LEDs.
Warning light arrays for emergency vehicles and aircraft may require coordinated light emission from several light sources arranged at various points on the vehicle or aircraft. This has typically been accomplished by use of a central controller. The central controller stores light emission patterns which are applied in a coordinated manner to energize the several light sources to produce the coordinated pattern of light emission. Such pre-existing warning light arrays may employ prior art light sources such as halogen bulbs and/or xenon flash tubes as emitters. Many operators of fleet vehicles and aircraft are desirous of replacing the prior art light sources to take advantage of the superior durability and energy efficiency of LED light sources. Thus, there is a need in the art for LED driver circuits responsive to control signals from a central controller to energize an LED light source, such that the prior art light sources may be replaced without requiring replacement of the controller and/or the control wiring.